Styling and performance requirements have directed the automotive industry toward headlamps with clear lenses and reflector optics. Further, demand for increased headlamp performance has heightened interest in high intensity discharge (HID) light sources. Marriage of these two technologies into one product requires that several technical problems be overcome.
Two of the most difficult issues in designing reflector optics for a low-beam headlamp for use with an HID source are control of glare light and control of excess amounts of foreground light. These problems stem from the extended discharge area of the HID source, which is an ellipse of about 4.4 mm.times.4.4 mm.times.7.1 mm, compared to the typical tungsten halogen (TH) bulb, which is a cylinder 5.2 mm.times.1.2 mm. The extended discharge area of the HID source creates a large effective light source, which causes a large angular spread of light from a point on the reflector surface. This large angular spread can lead to excess amounts of glare light or foreground light.
The angular spread of light (i.e., the angular image size) from any point on the reflector depends on the size of the light source and the distance between the source and the point on the reflector. The image sizes produced by the reflector, however, can be controlled to some extent. For example, the image size can be decreased by increasing the distance between the light source and the point on the reflector.